1. Field of the Invention
The present invention relates to optical imaging systems in which imaging apparatuses such as video cameras and digital still cameras using two-dimensional solid-state imaging elements such as CCDs, and CMOS devices, are used.
2. Description of the Related Art
Hitherto, imaging optical systems which include solid-state imaging elements such as CCDs, CDMOS devices, and the like, wherein pixels are arranged two-dimensionally with a predetermined pitch, have been widely used in imaging apparatuses such as digital still cameras and video cameras.
In FIGS. 3 and 4 the structures of conventional optical imaging systems are shown. In these figures, reference numeral 31 represents an imaging lens, and reference numeral 32 represents an optical-low pass filter. An IR (infrared) cut coating 38 comprising a multi-layer film is formed on the surface of the imaging-lens side of the optical low-pass filter 32, and this optical low-pass filter 32 also acts as an IR cut filter.
Reference numeral 37 represents an imaging element such as a CCD, a CMOS device, or the like. On-chip microlenses 35 and a color filter 36 are disposed on the imaging-lens side of the imaging element 37 in that order when viewed from the imaging lens to the imaging element. This imaging element 37, the on-chip microlenses 35 and the color filter 36 are hermetically sealed by a package 33 and a sensor cover glass 34.
In optical imaging systems constructed in this manner, image rays incident upon the imaging lens 31 are condensed at the imaging lens 31, and are then further condensed towards each of the light-receiving portions (image pixels) of the imaging element 37 by the on-chip microlenses 35. Only colored light divided into each RGB component by the color filter 36 will efficiently strike the individual light-receiving portions of the imaging element 37.
As shown in FIG. 5, the color filter 36 comprises a red-transmitting filter (R) 61, a green-transmitting filter (G) 62, and a blue-transmitting filter (B) 63, which are formed to correspond to each of the light-receiving portions of the imaging element 37. For each of the filters 61 to 63, material that transmits light selectively according to its wavelength is provided, and the spectral characteristics thereof are as shown in FIG. 6. FIG. 6 shows the spectral characteristic of the blue-transmitting filter 63, indicated by reference numeral 65; the green-transmitting filter 62, indicated by reference numeral 66; and the red-transmitting filter 63, represented by reference numeral 67. In each of the light-receiving portions, colored light, which has been divided into RGB, is incident upon the primary-color filters. Additionally, as shown in FIG. 5, the color filter 36 is arranged according to the Bayer pattern.
The conventional optical imaging systems discussed above can be used without problems under normal conditions. However, in cases where there is background light or when there is a strong light source shining on the screen, the IR cut coating 38 becomes responsible for the generation of unnatural red ghosting on the screen, and thus the image quality is reduced.
An example is the occurrence of ghosting in the light path shown in FIG. 3. When imaging at night, in cases where there is a strong light source like an electric light shining on the screen, a portion of light 101 directed toward the imaging element 37 is reflected by the IR cut coating 38 of the surface of the optical low-pass filter 32, is then reflected by one of the lenses in the imaging lens 31, and is further transmitted through the IR cut coating 38, becoming a ghost that is incident on the imaging element 37.
The following is an explanation of why the ghost takes on a red color. The IR cut coating 38 has the spectral transmittance shown in FIG. 7. However, the spectral reflectance at the time of reflection is the value obtained after subtracting this spectral transmittance from 100%, and therefore becomes a spectral characteristic that is inverted about the 50% line.
The spectral characteristic of the ghosting is calculated as follows:spectral transmittance×spectral reflectanceFIG. 8 shows a plot of the spectral characteristic of the ghosting for each wavelength. As shown in the figure, in the visible region of the spectrum where the characteristic is flat, and the transmittance of the IR cut coat 38 is near 100%, and in the infrared region of the spectrum and the ultraviolet region of the spectrum where the transmittance is near 0%, an extremely small value is obtained for the intensity of the ghosting when calculated using the above formula.
However, at wavelengths where the transmission of the IR cut coating 38 becomes 50%, the reflectance is also 50%. Therefore, an extremely large value of 25% is obtained for the intensity of the ghosting when calculated using the above formula.
When a ghost having this type of spectral characteristic is transmitted through the color filter 36 and impinges upon the light-receiving elements of the imaging element 37, since the transmittance through the color filter 36 is low for light that has a large intensity at short wavelengths, as shown in FIG. 6, the intensity of the ghost becomes extremely small.
In contrast, with regard to light having a large intensity at long wavelengths, as shown in FIG. 6, since the red-transmitting filter has a structure which does not prevent transmission at the infrared side, the spectral transmittance of the infrared light becomes high, and the intensity of the ghosting becomes extremely large. As a result, the image is recorded with a prominent ghost having a red tone.
Due to this, at the point-symmetrical position of the light source about the center of the image plane, red ghosting, which cannot be seen on silver halide photographs, appears.
One more type of ghosting is the type shown in FIG. 4. When imaging at night, in cases where there is a strong light source like an electric light, a portion of the light 102 toward the imaging element 37 is reflected by the surface of the on-chip microlenses 35, is subsequently reflected by the IR cut coating 38 on the surface of the optical low-pass filter 32, and is then transmitted through the optical low-pass filter 32, becoming a ghost that is incident on the imaging element 37.
With regard to the on-chip microlenses 35, since they have a structure in which the microlenses are regularly arrayed, they simultaneously diffract light as it is reflected at the surface of the on-chip microlenses 35, and a diffraction pattern is exhibited.
This ghost has a red tone for the same reason as the ghost caused by the reflection between the lens surface of the imaging lens 31 and the above-mentioned IR cut coating 38. Consequently, ghosting caused by the on-chip micro lenses 35 and the IR cut coating 38 is generated around the light source as a ghost having a red-toned diffraction pattern, which cannot be seen in silver halide photographs.
Furthermore, for IR cutting filters, in addition to the reflection types discussed above, there are absorption types that, if used, may be able to eliminate this type of ghosting.
However, in general, in the absorption-type IR cutting filters, the thickness in the optical axis direction is large compared to that of the reflection type, and therefore they are not preferable in light of the trends toward imaging apparatus miniaturization.